1. Field of the Invention
The present invention relates to a storage apparatus such as a magnetic disk apparatus, a control method, and a control device which positions a head on a target track of a rotating storage medium and causes the head to follow it and particularly, relates to the storage apparatus, the control method, and the control device which compensates for rotation disturbance vibrations applied to the apparatus so as to ensure head positioning accuracy.
2. Description of the Related Arts
Conventionally, in a magnetic disk apparatus, a head position signal is demodulated from servo information of a magnetic disk read from a head, and following control (also referred to as “on-track control”) which positions the head so that the head tracks the track center of a target track is carried out based on the head position signal. However, in a usage environment of the magnetic disk apparatus, it is put in a rack of, for example, a server or a storage system; therefore, it is affected by the vibrations of a fan or another magnetic disk apparatus. These vibrations physically vibrate a rotary actuator, and the vibrations appear in the head position signal as disturbance. The vibrations applied to the magnetic disk apparatus include translation vibrations and torsion vibrations. The translation vibrations are the vibrations that move the entire apparatus in one direction and do not affect head positioning since the rotary actuator also integrally vibrate. On the other hand, the torsion vibrations are the vibrations that moves the entire apparatus in a rotation direction about a certain position, become the vibrations that moves the rotary actuator rotatably supported by a pivot shaft in the rotation direction, and become disturbance that causes a head position error, which is called rotation disturbance vibrations. In order to eliminate the deterioration in the positioning accuracy due to such rotation disturbance vibrations, in a conventional magnetic disk apparatus, an acceleration velocity sensor, which detects the disturbance vibrations, is provided, and a servo compensation signal which is proportional to the output of the acceleration velocity sensor is added to a positioning control servo system so as to cancel them out; thus, even when unexpected disturbance vibrations from outside are applied, the head position error caused by the external vibration factor can be reduced (Patent document 1).    [Patent document 1] Japanese Patent Application Laid-Open Publication No. S63-213176    [Patent document 2] Japanese Patent Application Laid-Open Publication No. 2006-221732    [Patent document 3] Japanese Patent Application Laid-Open Publication No. H08-17151
However, in the conventional compensation control using the acceleration velocity sensor, the signals output from the acceleration velocity sensor may include noise components caused by the factors other than the rotation disturbance vibrations such as translation vibrations, power source ripples, and the S/N ratio of the sensor itself in some cases, and there is a problem that the positioning accuracy is deteriorated by the influence of the noise components contained in the compensation signals of the rotation disturbance vibrations added to the positioning control system. The acceleration velocity sensor used in the compensation control is intended to detect the rotation disturbance vibrations in the disk plane (X-Y plane) of the magnetic disk apparatus; however, in practice, there is a problem that the output of the acceleration velocity sensor includes signal components other than the rotation disturbance vibrations caused by power source ripple noise or translation in the direction (Z direction) orthogonal to the disk plane, appropriate sensor signals are fed forward to the positioning control system as a result, and the head positioning error is worsen. Even in a normal case in which no disturbance vibrations or noise are applied to the acceleration velocity sensor, the sensor signal contains noise, components due to the influence of the S/N ratio of the acceleration velocity sensor itself. There is also a problem that the positioning accuracy is worsen when such sensor signal is fed forward to the positioning control system.